The invention relates to a system for detecting holes on a magnetic medium or an optical medium especially for serpentine recording formats or parallel track formats using stationary read and write heads.
In high speed magnetic tape reading and writing units ("tape streamers"), data is read from, or written in, a plurality of data tracks which run parallel to the edges of a magnetic tape. Holes, patterns of holes or patterns formed by selectively creating semi-transparent areas in the tape are used for auxiliary information coding. In a conventional system for detecting holes, such as an end-of-media sensor, light emitting diodes (LED's) or incandescent lamps are used as sources of light. Light beams are directed towards the tape or other medium contained within a cassette. Discrete phototransistors are used as light sensitive devices and as pre-amplifiers. These phototransistors are placed in nominal positions where the light beams from the emitters are expected to strike when the tape holes on the tape pass.
A problem associated with such a system is that a preamplifier gain adjustment procedure is required to set the signal level input to a fixed threshold amplifier or comparator. This is normally performed using a potentiometer. Design of a fixed preamplifier gain is difficult and costly to implement for volume production since differences in optical/electrical DC transfer ratios are very large. The difference in light output is generally always greater than 4:1 for the emitters, typically 6:1 when the max/min values are specified using standard parts. The emitted light will also vary with temperature, for example, a ratio of 1.3:1 is typical.
For phototransistors, the gain spread is typically specified at 2:1 for a fixed temperature and is often more for standard parts. Furthermore, the dark current of phototransistors varies strongly With temperature which may easily cause a drift in the 100 mV-range in the DC output for worst-case samples of detectors with large collector loads. The calculations are further complicated by the mechanical tolerances of the optical path in the system including the tolerances of the placement of the emitters and receivers. As a result of the need for mass production of low-cost discrete components having large noise margins in a reasonable dynamic range, the detector in a typical tape streamer is built for operation using a +12 volt supply.
Integrated sensor systems are available which are sensitive to stray light pick-up which often is very difficult to eliminate in certain systems. Such systems have threshold levels which are specified for DC light inputs, and therefore, small margins exist for noise pick-up. However, relatively large currents may be needed in some applications to drive the light emitters to obtain a good signal-to-noise ratio. The threshold level will typically vary with temperature, from .+-.10% to .+-.20%, depending on the operating temperature range.
To reduce stray light pick-up problems and to increase the efficiency, infrared emitters and visible light cut-off filters are normally used. Applications which require even more ambient noise suppression include synchronous demodulators on the integrated sensor chip. The sensors will also have a specification for the ambient illuminance.
In addition, an oscillator and a driver for the light emitting diode is included in the synchronous receiver which may be disadvantageous in applications where the transmitter and receiver are physically separated. In addition, a problem with synchronous detectors exists in that excessive time delays in phase shifts may occur for the light emitting diodes. Depending on the type of emitters used, the phase shift may vary over the range 10 to 20 kHz. To avoid this, a carrier regenerator must be built into the receiver to reliably demodulate the signal. For many applications, for example, magnetic tape hole detection, a frequency below 10 kHz used by the synchronous detectors will be too low, since the tape often runs at a speed of 120 IPS (inches/second).